1. Technical Field
The present invention relates to a printed circuit board and more particularly to a technique for fabricating a printed circuit board so as to prevent the flow of molten solder into a via during the process of wave soldering the bottom surface of the circuit board.
2. Related Art
A printed circuit board typically accommodates a surface mount device such as a ball grid array (BGA) module. A BGA module comprises an array of solder balls located on the underside of the BGA module. The top surface of a printed circuit board contains a landing area comprising a corresponding array of top pads. When a BGA module is installed on the landing area of the top surface, each solder ball rests on a top pad and is affixed to the top pad by means of a solder joint. Each such top pad is electrically connected to a via. The via is a cylindrical hole extending through the thickness of the circuit board and is lined with a material, such as copper, that acts as both an electrical and a thermal conductor. The via provides a path of electrical conduction between the BGA module and electrical circuit patterns located within internal layers of the circuit board.
After BGA modules are installed on the top surface of a circuit board, pin-in-hole components such as connectors, SIPS, and DIPS are placed on the top surface of the circuit board and are affixed by wave soldering the bottom surface of the circuit board. During the wave soldering of the bottom surface, the circuit board is transported over a molten bath of solder, wherein upward movement of solder through a chimney causes a fountain of solder to be drawn into pin holes so as to encase leads of pin-in-hole components. Unfortunately, the molten solder also travels up the vias. The hot solder within a via transfers substantial heat to its associated top pad because of a large thermal conductance between the via and the pad. Accordingly, the solder joint at the top pad is susceptible to reflow (liquidization and redistribution) from the applied heat, which may cause an open circuit. Alternatively, the heat and consequent reflow may degrade the integrity of the solder joint, thereby making the solder joint susceptible to fracture from stress during subsequent usage of the circuit board.
In order to avoid the expense and time delay associated with rework and repair due to impaired solder joints resulting from wave soldering, it is necessary to block the transfer of heat from the molten solder to the top pads. The prevailing method of inhibiting such heat transfer is to cover the via opening at the bottom surface of the circuit board with insulating material such as KAPTON(trademark) (a trademark of DuPont) tape. See U.S. Pat. No. 5,511,306 (Denton et al., Apr. 4, 1994) which is hereby incorporated by reference. The technique of covering the via opening has the disadvantages of adding steps to the board-fabrication process and incurring the cost of materials such as the KAPTON tape.
Thus, there is a need for an efficient and inexpensive method of preventing hot molten solder from flowing up through a via during the wave soldering process in order to inhibit heat transfer from the molten solder to a top pad.
The present invention provides a method of inhibiting the transfer of heat, during a wave soldering of the bottom surface of a printed circuit board, from the molten solder to a solder joint at a top pad on the top surface. The present invention creates a mechanical plug in the via prior to the wave soldering in order to obstruct the bottomside flow of molten solder into the interior of the via. By preventing the molten solder from transferring heat to the conductive lining of the via, the plug shields the top pad from heat that might otherwise cause degradation of the solder joint at the top pad.
Affixation of SMT components onto the bottom surface of the printed circuit board is accomplished by screening solder paste onto bottomside pads, dispensing adhesive onto the bottom surface in the vicinity of the solder paste, placing each component on the bottom surface by placing the component body on the adhesive and the component lead on the solder paste, and reflowing the solder paste and adhesive. This provides a mechanical and electrical connection between the bottomside components and the bottom surface. The present invention enables the plug to be formed as a part of this screening and reflow step. This is accomplished by having the screen comprise apertures corresponding to both the bottom pads and the vias. In this manner, solder paste is screened into the vias at the same time that solder paste is screened onto the bottom pads. The reflow step creates plugs of solder in the vias by redistributing the solder paste within the vias at the same time that the solder paste on the bottom pads are reflowed to form solder joints.
After completion of the bottomside process, BGA modules and other components may be affixed to the top surface of the circuit board by use of another screening and reflow process which creates solder joints at the pad-component interfaces on the top surface of the circuit board. Next, pin-in-hole components on the top surface, as well as additional components previously mounted on the bottom surface, may be affixed to the circuit board by wave soldering. During the wave soldering of the bottom surface, the solder joints on top pads attached to vias are adequately protected from thermal degradation by the insulating effect of the plugged vias.
A solder plug in a via is effective for the present invention because the plug prevents molten solder from entering the via and the plug has a high thermal resistance to the conductive transport of heat. Nevertheless, because the solder plug is a good electrical conductor, the plug may serve as an electrical contact point for placing probes and the like during a testing phase after the circuit board assembly is completed. The plug provides the additional advantage of preventing oxidation within the via lining. This is especially valuable, because the material that lines the via, typically copper, is subject to oxidation and an oxidized via lining is ineffective for conducting electricity to an applied instrumental probe. Although the via lining is typically coated with a material that inhibits oxidation, this protective coating can be burned off during the heating phases (e.g., during reflow) of circuit board assembly. The consequent oxidation of the via lining after loss of the protective coating places an upper limit on circuit board assembly time. The solder plug of the present invention circumvents these problems by inhibiting oxidation of the via lining and providing an accessible electrical contact surface for an applied probe.
Thus, the present invention has several advantages. The present invention provides a low cost, time-efficient method of preventing thermal degradation of solder joints at top pads that are connected to vias. The present invention also eliminates the extra step inherent in the prior art method of taping the via opening. Additionally, the present invention inhibits oxidation of the via lining, thereby providing electrical contact surfaces for probing the printed circuit board at the vias during testing of the circuit board following assembly.